Method of making corrugated thermoplastic articles

ABSTRACT

CORRUGATED THERMOPLASTIC ARTICLES ARE MADE BY SIMULTANEOUSLY PERFORATING IN CLOSELY SPACED PARALLEL ROWS AT LEAST TWO UNIAXIALLY ORIENTED SHEETS OF THERMOPLASTIC POLYMER IN INTIMATE CONTACT AT AN ANGLE OF AT LEAST ABOUT 10* RELATIVE TO ORIENTATION WITH HEAT TO AT LEAST FUSION TEMPERATURE OF THE TERMOPLASTIC, COOLING FUSED PERIPHERIES OF THE PERFORATIONS AND ANNEALING THE PERFORATED FILMS.

P. H. PARKER 3,580,769

METHOD FOR MAKING CORRUGATED THERMOPLASTIC ARTICLES May 25, 1971 Original Filed April 5, 1968 INVENTOR PHIL L/P H. PARKER United States Patent 3,580,769 METHOD OF MAKING CORRUGATED THERMOPLASTIC ARTICLES Phillip H. Parker, San Rafael, Califi, assignor to Chevron Research Company, San Francisco, Calif. Original application Apr. 5, 1968, Ser. No. 719,147. Divided and this application Jan. 19, 1970, Ser.

Int. Cl. B311? 1/22 US. Cl. 156-206 11 Claims ABSTRACT OF THE DISCLOSURE CROSS-REFERENCE TO RELATED APPLICATION This application is a division of copending application Ser. No. 719,147, filed Apr. 5, 1968.

FIELD OF INVENTION This invention concerns corrugated thermoplastic articles made from uniaxially oriented sheets of substantially crystalline polymer and methods for preparing such articles.

INVENTION DESCRIPTION The corrugated thermoplastic articles of this invention comprise at least two uniaxially oriented sheets of substantially crystalline polymer. The sheets are cross lapped at an angle of at least about relative to their directions of orientation, perforated in closely spaced, parallel rows and fusion bonded at the peripheries of the perforations. Each row of perforations runs approximately along the bottom of the corrugation grooves.

Thermoplastic polymers which are useful in this invention include orientable, normally solid, substantially crystalline poly-l-alkenes such as polypropylene, polyethylene, poly-l-butene and poly-4-methyl-1-pentene, polyesters, polyamides, polyvinyl chloride, polystyrene, polycarbonates and the like. The term polymers as used herein includes copolymers and physical mixtures of the above-described thermoplastics. Poly-l-alkenes of monomers of 2 to 6 carbon atoms are preferred. Polypropylene is particularly preferred. If sheets of different polymers are used, they must be fusible thermally.

The general technology'for preparing the sheets used in this process is well known in the art. The substantially crytalline polymer is extruded in a conventional extruder to provide a sheet or film of desired thickness and width. These sheets or films have substantially uniform thicknesses. The film is then oriented uniaxially 'by stretching it in one direction at elevated temperatures. The stretch temperatures will depend on the polymer used. The film may be heated to the desired temperature by passing it through a heating zone or over heated rolls or drums. It may be stretched in a single step or in a plurality of steps. The draw ratios used to orient the film will normally be in the range of about 4:1 and the ratio which causes the film to fibrillate spontaneously. This latter ratio again will depend upon the particular polymer involved. For the preferred polymer, substantially crystalline polypropylene, draw ratios between about 6:1 and 12:1 will be used. Preferably the oriented sheet or film thickness will be in the range of about 0.5 and about 5 mils.

3,580,769 Patented May 25, 1971 The sheets are positioned such that their orientation directions are not parallel but are rotated at least about 10 relative to each other. In other words they are positioned so that their lines of orientation intersect at an angle of at least about 10. Such positioning gives the corrugated article better lateral strength than if the lines of orientation of the sheets were aligned parallel. Preferably the sheets are rotated at approximately to each other.

While articles containing any number of sheets may be prepared, articles made with two to four sheets will be most common. Articles made with two sheets are preferred.

The shape of the perforations is a practical rather than a critical variable. They may be square, oblong, triangular, rectangular, circular, etc. They will normally be circular. These circular perforations will usually be about 0.01 to about 1 inch in diameter. As regards the spacing of the perforations adjacent perforations in a row will normally be about 0.1 to about 3 inches apart and the rows of perforations will normally be about 0.1 to about 3 inches apart. Preferably, the distance between adjacent rows and the distance between adjacent perforations in a row will be substantially the same.

The articles of this invention are made by first placing two or more thermoplastic sheets in intimate contact at an angle of at least about 10 relative to their directions of orientation. The films are then perforated simultaneously with a sharp element, such as a needle, heated to at least about the fusion temperature of the thermoplastic. For polypropylene the elements will usually be heated in the range of about 300 F. to about 600 F. The size and shape of the element is chosen to correspond to the desired perforation dimensions and shape. The puncturing element will be made from a material which does not stick substantially to the heated thermoplastic. As the element punctures the films the heat causes the films to melt and fuse at the periphery of the perforation. Closely spaced parallel rows of such perforations are thus made by using a single element repeatedly or a multiplicity of elements operating in unison. After the perforation is formed the element is withdrawn from the hole and the fused periphery is allowed to cool and solidify.

After the fused peripheries cool the hole-bonded sheets are annealed. The annealing conditions should be such that the sheets shrink at least about 5%. The amount of shrinkage along with the row spacing affects the amplitude of the corrugated ridges. The annealing temperature will be between the glass transition temperature and melting point of the polymer involved. The particular temperature used may also depend on the thermal and mechanical history of the polymer. A substantially crystalline polypropylene film uniaxially oriented to an 8:1 draw ratio shrinks 9% at 250 F., 15% at 275 F. and 43% at 300 F. The annealing may be done conveniently by placing the bonded sheets in a heated oven or bath for the desired time. The shrinkage will usually take less than about 2 minutes; however, longer annealing times may be used if desired.

If the laminated film is used for packaging materials not subject to heat degradation, the laminated film may be first formed around the material and then annealed.

The articles of this invention may be more fully understood by referring to the attached drawing. This drawing illustrates a section of a corrugated article made from two sheets, A and B, cross lapped such that their directions of orientation are rotated 90. Sheet As direction of orientation runs horizontally and Bs runs vertically. The 'vertical rows of perforations, 1, run along the bottom of the grooves of sheet As corrugations. correspondingly,

the horizontal rows of perforations, 2, run along the grooves of sheet Bs corrugations.

EXAMPLES The following examples illustrate the corrugated articles 4 sheets of thermoplastic polymer which are in intimate contact at an angle of at least about 10 relative to their orientation with elements heated to at least the fusion emperature of the thermoplastic, cooling the fused peripheries of the perforations and annealing the perforated of this invention and the method used to make them. 5 fil These examples are in no manner intended to limit the 2, M th d f l i 1 he ein the polymer is a subinvention described herein. stantially crystalline poly-l-alkene of monomers having Corrugated articles of this invention were prepared 2t06 rbon at ms, using the following general procedure. 1 ft. square pieces 3. Method of claim 1 wherein the polymer is substanof 1.9 mil thick uniaxially oriented polypropylene films tially crystalline polypropylene. (draw ratio of 8:1) were cross lapped at 90 relative to 4. Method of claim 1 wherein the number of sheets is their orientation directions. The films were perforated in in the range of 2 and 4, inclusive. the desired pattern by repeatedly placing the circular tip 5. Method of claim 1 wherein the number of sheets is 2. of a 40 watt soldering iron at 450 F. to 500 through 6. Method of claim 1 wherein the sheets are 0.5 to 5 the films. Care was taken to hold the films close together mils thick. so that the melted polypropylene of both films was in 7. Method of claim 1 wherein the angle is about 90. contact. The hold-bonded films were then annealed by 8. Method of claim 1 wherein adjacent perforations placing them in a forced draft oven at the desired temin a row are about 0.1 inch to 3 inches apart and the rows perature for one-half hour. The strength and elongation are about 0.1 inch to 3 inches apart. properties of the resulting corrugated sheets were eval- 9. Method of claim 1 wherein the distance between aduated by ASTM D 1682-64. For comparison, similarly jacent perforations in a row is substantially the same as prepared hole-bonded films which were not annealed were the distance between adjacent rows. also evaluated for strength and elongation. The particulars 10. Method of claim 1 wherein the elements are cirof these preparations and evaluations are reported in the cular and have diameters in the range of about 0.01 to table below. 1 h.

Laminate construction Hole spacing Between rows of Between holes Article evaluation test Number es, in row, in. of film Hole Breaking Example layers in size, Ver- Hori- Ver- Hori- Annealing Breaking elongation, Number laminate mm. tical zontal tical zontal Holepattern temp., F. load, 1b. percent 2 1 0. 25 0.25 0. 25 0. 25 Square Not annealed- 90 9 2 1 0. 25 0. 25 0. 25 0. 25 ..---do 200 105 12 2 1 0. 25 0. 25 0. 25 0. 25 do 250 128 13 2 1 0.5 0.5 0. 25 0. 25 Staggered--. Not annealed 2 1 0. 5 0. 5 0. 25 0. 25 d 200 95 13 2 1 0. 5 0. 5 0. 25 0. 25 107 18 2 1 0.5 0.5 0.5 0.5 77 9 2 1 0. 5 0. 5 0. 5 o. 5 do 126 21 2 2 0.25 0.25 0. 25 o. 25 --do Not annealede0 9 2 2 0. 25 0. 25 o. 25 0. 25 do 200 5s 10 2 2 0. 25 0. 25 0. 25 0. 25 d 22 1s 2 2 0.5 0.5 0. 25 0. 25 e2 7 2 2 0. 5 0. 5 0. 25 0. 25 75 11 2 2 0.5 0.5 0. 25 0. 25 do 25o s4 16 2 2 0. 5 0. 5 0. 5 0. 5 Square Not annealed 74 10 2 2 0.5 0.5 0.5 0.5 do 250 110 19 2 1 0. 25 0. 25 0. 25 0. 25 do 25o.-. 227 25 The above table shows that the corrugated articles of 11. Method of claim 1 wherein the polymer is polythis invention have better strength and elongation properpropylene, the elements are heated in the range of 300 'F. ties than corresponding non-corrugated laminates. Corand 600 F. and the films are annealed at 225 F. to rugated articles of this invention are useful in packaging 300 F. applications such as shrink packaging where the corruga- References Cited tion may be fomed during packaging. UNITED STAT PA As will be evident to those skilled in the art, various ES TENTS modifications on this invention can be made or followed, 3,93%453 5/1962 Ford 156253X in the light of the foregoing disclosure and discussion, 3520044 5/1967 Cole et 156206X without departing from the spirit or scope of the disclosure or from the scope of the following claims. BENJAMIN RPADGE'IT Pnmary Exammer I clai S. I. LECHERT, 'JR., Assistant Examiner 1. Method for making a corrugated thermoplastic ar- US. Cl. X.R.

ticle which comprises simultaneously perforating in closely spaced parallel rows at least two uniaxially oriented 

